Remote sensing, i.e. for instance to monitor particular aerosol concentrations in the Earth's atmosphere is carried out by an imaging satellite detector that captures multiple images of the Earth and calculate from these images a Degree of Linear Polarization (DoLP), the Angle of Linear Polarization (AoLP) and the radiometric intensity.
Satellites are orbiting the Earth at an altitude of typically between 400 and 800 km. These orbits are indicated as Low Earth Orbits (LEO). The speed of these satellites relative to the Earth is about 7 km/s. Since an integration time of about 1s is needed to arrive at a good enough Signal to Noise Ratio (SNR), an effective ground pixel on Earth will be about 5 km in flight direction. Most often some binning in the swath direction is used leading to square ground pixels of e.g. 5×5 km.
Scientists are asking for smaller ground pixels while keeping good SNR values. This is difficult since the scattering by the Earth is a constant, as is the output of the Sun, so the only way to decrease the ground pixel size is by moving towards larger entrance apertures and smaller f-numbers in the optical design. This leads to larger, heavier, and more expensive instruments.
An example of such an aerosol detection system by multi-polarisation imaging in a satellite application is found for instance in “The MetOP Second Generation 3MI instrument”, Ilias Manolis, Proc. of SPIE Vol. 8889 88890J-1. Overlapping 2D images on the surface of Earth are recorded consecutively at regular points along an orbit and thus providing the means to sense the Top of Atmosphere radiance at different Observation Zenith Angles for each target. In this way ground pixels are measured at many angles and the angular distribution of the DoLP and intensity, and many characteristics of an aerosol distribution can be determined. In the disclosed device spectral channels within each module are recorded sequentially, while, for the polarized ones, three consecutive polarization measurements are taken with a linear polarizer oriented at +60, 0, and −60 degrees respectively for each channel.
The prior art device relies on a broadband telecentric design, wherein prior to detection a telecentric beam is projected through a spectral filter and polarization filters, the telecentric design ensuring a controlled optical functionality of the filters, in order to provide a reliable—per pixel detection of polarization.
A drawback of the prior art imaging device is that it relies on an extremely large aperture of the first lens in order to have a sufficient angle of view. This is necessary since in flight direction a number of subsequent measurements are carried out for different polarizations and colors, with the same equipment. This reduces the level of accuracy of the polarization detection, and renders the device very vulnerable for deterioration since the input is a wide angle lens of several centimeters. In relation therewith, the optical design is complex since it is designed for many wavelengths that are measured sequentially. This puts a high demand on the chromaticity of the system.
Instead of sequential imaging, another approach is to instantaneously measure a polarization state of the incoming light beam in a parallel measurement. For example, this beam may be distributed via a power splitter over multiple channels. In those channels, light can then be split into the s and p-polarized component. This will result in plural images of the earth, the one for s-, the other for p-polarized light. For example another output of the power splitter can be rotated 45 degrees in polarization and then also have to be dissolved in s- and p-components. From these four polarization stepped images, a degree of linear polarization can be determined, a direction of polarization, and the intensity.
With this approach the aim is to provide a better design of a spatially resolved polarization detector with lower input aperture in particular smaller than 4 mm which fulfills the criteria of compactness, where a desire exists to carry out the measurements in parallel for multiple wavelengths to obviate the problem of limited integration time for obtaining sufficient spatial resolution. Thus the problem is to provide an optical design that can be carried out in parallel, wherein the input aperture is limited to a value substantially smaller than 4 mm. This has an advantage that the optics can be optimized for a specific spectral range, so that polarization can be preserved and better accuracies can be obtained. It is also desired to provide a design that can easily be calibrated in space, which is non-trivial due to the working conditions.